Microminiature stirling cycle cryocoolers and engines

ABSTRACT

A microminiature Stirling cycle engine or cooler is formed utilizing semiconductor, planar processing techniques. Such a Stirling cycle thermomechanical transducer has silicon end plates and an intermediate regenerator. The end plates are formed with diaphragms and backspaces, one end plate forming the expansion end and the opposite end plate forming the compression end, with the regenerator bonded in between. A control circuit apparatus is linked to the diaphragms for controlling the amplitude, phase and frequency of their deflections. The control circuit apparatus is adapted to operate the transducer above 500 Hz and the passages and the workspace, including those within the regenerator, expansion space and compression space, are sufficiently narrow to provide a characteristic Wolmersley number, which is characteristic of the irreversibilities generated by the oscillating flow of the working fluid in the workspace, below substantially 5 at the operating frequency above 500 Hz. Additionally, the amplitude of the vibrations of the diaphragm vibrations are sufficiently small to provide the working fluid a maximum Mach number below substantially 0.1 at an operating frequency above 500 Hz.

This is a division of application Ser. No. 08/541,260, filed Oct. 12,1995, now U.S. Pat. No. 5,749,226, which is a continuation-in-part ofapplication Ser. No. 08/333,356 filed Nov. 2, 1994, Now U.S. Pat. No.5,457,956, which is a continuation of application Ser. No. 08/190,582filed Feb. 2, 1994, now abandoned, which is a continuation ofapplication Ser. No. 08/017,265 filed Feb. 12, 1993, now abandoned.

TECHNICAL FIELD

This invention relates generally to Stirling cycle engines andcryocoolers and more particularly relates to such Stirlingthermomechanical transducers which are particularly useful for coolingintegrated electronic circuits to cryogenic temperatures and as enginesfor driving micromachines.

BACKGROUND ART

Because of the advantageous electronic properties exhibited by variousmaterials at cryogenic temperatures, various machines have beendeveloped for cooling electronic devices so that they may be operated atcryogenic temperatures. Many such refrigeration systems have usedStirling cycle cryogenic coolers. Such existing machines, however, arerelatively large, bulky, inefficient and noisy machines generatingsubstantial vibration. While technology has developed to permit aremarkable miniaturization of electronic circuitry, thereby permittinglarge numbers of electronic circuits to be contained within a relativelysmall volume, the apparatus which is available for cooling such circuitsis relatively large and consequently adds a substantial, additionalvolume and weight to cryogenic electronic circuitry. There is,therefore, a need for an efficient Stirling cycle cryocooler which canbe miniaturized so that its size and weight are compatible with and donot add substantially to that of the electronic circuitry but are,nonetheless, capable of pumping heat at a sufficient rate to maintainthe cryogenic temperatures.

A measure of the size and effectiveness of a cooler in pumping thermalenergy is its specific capacity. Specific capacity is the ratio of thequantity of thermal energy which the machine can pump to a quantitativemeasure of its size or weight. Thus, a cryocooler must not only be ableto pump sufficient thermal energy from the electronic device to maintainit at a cryogenic temperature, but should do so with the smallestpossible size or weight. Consequently, the higher the specific capacity,the more desirable is the cooler.

The prior art has recognized that the specific capacity of a Stirlingcycle cooler can be increased and therefore improved by operating thecooler at a higher frequency. A sufficiently high, thermal energypumping rate can be maintained if the cooler is made smaller, but thefrequency of its operation is increased so that more thermal energypumping cycles occur each second.

However, the prior art has also recognized that entropy generatingprocesses (ie, irreversibilities) have imposed an upper limit on thefrequency of operation of Stirling cycle machines. As the operatingfrequency is increased, viscous dissipation resulting from the frictionof the working fluid with the internal passage walls of the Stirlingcryocooler also increases. As a result, more work is required to movethe gas back and forth through the passages of the Stirling cyclemachine at higher frequencies. In addition, the apparent thermalconductivity of the working fluid in the regenerator increases causing alarger amount of heat to be conducted into the cold end of the machine,and heat transfer in heat exchangers is reduced. The latter effectsoccur as a result of certain phase relationships between the periodicvariations in axial mass flow and radial temperature gradient whicharise in oscillatory flow. As a result, the amount of heat that must bepumped by the cryocooler is increased while the effectiveness of thecryocooler in exchanging heat with its surroundings is reduced.Consequently, increasing the frequency reduces the heat lifted.Therefore, the prior art has come to accept an upper frequency limit forStirling cycle machines on the order of 50 Hz and a machine constructedto operate at 120 Hz is believed to be the highest frequency Stirlingmachine ever built.

There is, therefore, a need for a Stirling cryocooler which can liftheat at a sufficient rate to maintain electronic devices at cryogenictemperatures, but which also has a sufficiently small size and weightthat it exhibits a specific capacity which is acceptable and compatiblewith the equipment which utilizes these electronic circuits.

Recent years have also seen the development of a micromachinetechnology. While such machines utilize mechanical devices, such asmotion conversion linkages, mechanical advantage mechanisms, powertrains, valves, diaphragms, cantilever beams and the like, which haveconfigurations and modes of operation like conventional mechanicaldevices, they have a size on the order of a few millimeters or smaller.

Although Stirling cycle engines have long been used as mechanical primemovers, there is a need for a microminiature Stirling cycle engine foruse with developing micromachine technology.

BRIEF DISCLOSURE OF THE INVENTION

The prior art discloses Stirling cycle, thermomechanical transducershaving a pressure containing vessel, including fluid passages, andcontaining a compressible and expansible working fluid. The prior artalso discloses the use of flexible diaphragms associated with thetransport of the working fluid and the volume changes which are inherentwith the Stirling cycle. Such diaphragms are combined with a controlcircuit apparatus or other means linked to the diaphragms forcontrolling the mode, amplitude, phase and frequency of theirdeflection.

The present invention utilizes the combination of certain masses andsprings associated with the diaphragms, and pressure forces arisinginside the transducer to controllably operate the Stirling cyclethermomechanical transducer above an operating frequency of 500 Hz, suchas 1000 Hz for example, combined with passages in the work space whichare sufficiently narrow to provide a Wolmersley number below 5 anddiaphragm amplitudes sufficiently small to provide Mach numbers below0.1 at the operating frequency. These narrow passages and smalldiaphragm vibration amplitudes sufficiently reduce the irreversibilitiesas work done in transporting the fluid and temperature drops intransporting heat to provide a machine which has an acceptably high,specific capacity. The result is that practical, sufficiently highspecific capacities can be achieved at much smaller sizes thanpreviously thought possible.

The invention also contemplates forming the heat regenerating region aswell as the heat accepting and heat rejecting region of the Stirlingcycle device of silicon and utilizing planar processing techniques suchas photolithography, etching, oxidation, bonding and thin filmtechnology to allow wafer-scale manufacturing, creating hundreds ofmicrorefrigerator components simultaneously. The invention furthercontemplates the construction of arrays of such transducers and theirconstruction both as coolers and engines. Further aspects of theinvention are described below or will become apparent to those skilledin the art.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view in perspective and in section illustrating an array ofStirling cycle transducers embodying the present invention for purposesof cooling electronic integrated circuits.

FIG. 2 is an enlarged view of a portion of FIG. 1 for illustratingdetail.

FIG. 3 is an enlarged view of another portion of FIG. 1 for illustratingdetail.

FIG. 4 is a view in horizontal section taken substantially along thelines 4--4 of FIG. 1.

FIG. 5 is a view in vertical section taken along the line 5--5 of FIG.1.

FIG. 5A is an enlarged view in vertical section of a portion of thestructure illustrated in FIG. 5.

FIGS. 6, 7, and 7A are views similar to those of FIGS. 4, 5, and 5A, butillustrating an alternative embodiment of the invention.

FIGS. 8 and 9 are views similar to those of FIGS. 4 and 5, butillustrating alternative embodiments of the invention.

FIGS. 10 and 11 are views similar to those of FIGS. 4 and 5, butillustrating alternative embodiments of the invention.

FIG. 12 consists of four graphs illustrating the displacement and volumevariations of an embodiment of the invention.

FIGS. 13-17 are block and schematic diagrams illustrating a controlsystem for the present invention.

FIG. 18 is a view in axial section illustrating a Stirling cycletransducer embodying the present invention for purposes of coolingelectronic and other devices.

FIG. 19 is a view in axial section illustrating an alternativeembodiment of the present invention as a Stirling cycle transducer forpurposes of pumping a fluid.

FIG. 20 is a view in axial section illustrating an alternativeembodiment of the present invention as a duplex Stirling cycletransducer for purposes of cooling electronic and other devices.

FIG. 20A is a view in cross section taken along the line A-A' of FIG.20.

FIG. 20B is a view in vertical section taken along the line B-B' of FIG.20A.

FIG. 21 is a view in axial section illustrating an alternativeembodiment of the present invention as a Vuilleumier heat pump forpurposes of cooling electronic and other devices.

FIG. 22A is a plan view of one side of a <110> silicon wafer.

FIG. 22B is a plan view of a cavity anisotropically etched into the sideof the wafer shown in FIG. 22A.

FIG. 22C is a vertical section taken along the line 1-1' of FIG. 22B.

FIG. 22D is a vertical section taken along the line 2-2' of FIG. 22B.

FIG. 22E is a plan view of another cavity anisotropically etched intothe side of the wafer shown in FIG. 22A.

FIG. 22F is a plan view of the other side of the <110> silicon wafershown in FIG. 22A.

FIG. 22G a plan view of a cavity anisotropically etched into the side ofthe wafer shown in FIG. 22F.

FIG. 22H is an oblique view of the cavity shown in FIG. 22B.

FIG. 23 is a view in perspective of one side of a representative portionof the preferred embodiment of a regenerative displacer chip.

FIG. 24 is a view in perspective of the opposite side of therepresentative portion of the regenerative displacer chip shown in FIG.23.

FIG. 25 is a view in perspective of one side of a representative portionof an alternative embodiment of a regenerative displacer chip.

FIG. 26 is a view in perspective of the opposite side of therepresentative portion of the regenerative displacer chip shown in FIG.25.

FIG. 27A shows vertical sections taken along the lines 1A-1A' and 2A-2A'of FIG. 24 and FIG. 23, respectively.

FIG. 27B shows vertical sections taken along the lines 1B-1B' and 2B-2B'of FIG. 24 and FIG. 23, respectively.

FIG. 27C shows vertical sections taken along the lines 1C-1C' and 2C-2C'of FIG. 24 and FIG. 23, respectively.

FIG. 28A shows an embodiment of the regenerative displacer formed bybonding together three of the dice shown in FIG. 24 and FIG. 23,respectively, and three dice that are the mirror images of those shownin FIG. 24 and FIG. 23, respectively, in a vertical section taken alongthe lines 1A-1A' and 2A-2A', respectively.

FIG. 28B shows an embodiment of the regenerative displacer formed bybonding together three of the dice shown in FIG. 24 and FIG. 23,respectively, and three dice that are the mirror images of those shownin FIG. 24 and FIG. 23, respectively, in a vertical section taken alongthe lines 1B-1B' and 2B-2B', respectively.

FIG. 28C shows an embodiment of the regenerative displacer formed bybonding together three of the dice shown in FIG. 24 and FIG. 23,respectively, and three dice that are the mirror images of those shownin FIG. 24 and FIG. 23, respectively, in a vertical section taken alongthe lines 1C-1C' and 2C-2C', respectively.

FIG. 29A shows vertical sections taken along the lines 1D-1D' and 2D-2D'of FIG. 24 and FIG. 23, respectively.

FIG. 29B shows vertical sections taken along the lines 1E-1E' and 2E-2E'of FIG. 24 and FIG. 23, respectively.

FIG. 29C shows vertical sections taken along the lines 1F-1F' and 2F-2F'of FIG. 24 and FIG. 23, respectively.

FIG. 29D shows vertical sections taken along the lines 1G-1G' and 2G-2G'of FIG. 24 and FIG. 23, respectively.

FIG. 30A shows an embodiment of the regenerative displacer formed bybonding together three of the dice shown in FIG. 24 and FIG. 23,respectively, and three dice that are the mirror images of those shownin FIG. 24 and FIG. 23, respectively, in a vertical section taken alongthe lines 1D-1D' and 2D-2D', respectively.

FIG. 30B shows an embodiment of the regenerative displacer formed bybonding together three of the dice shown in FIG. 24 and FIG. 23,respectively, and three dice that are the mirror images of those shownin FIG. 24 and FIG. 23, respectively, in a vertical section taken alongthe lines 1E-1E' and 2E-2E', respectively.

FIG. 30C shows an embodiment of the regenerative displacer formed bybonding together three of the dice shown in FIG. 24 and FIG. 23,respectively, and three dice that are the mirror images of those shownin FIG. 24 and FIG. 23, respectively, in a vertical section taken alongthe lines 1F-1F' and 2F-2F', respectively.

FIG. 30D shows an embodiment of the regenerative displacer formed bybonding together three of the dice shown in FIG. 24 and FIG. 23,respectively, and three dice that are the mirror images of those shownin FIG. 24 and FIG. 23, respectively, in a vertical section taken alongthe lines 1G-1G' and 2G-2G', respectively.

In describing the preferred embodiment of the invention which isillustrated in the drawings, specific terminology will be resorted tofor the sake of clarity. However, it is not intended that the inventionbe limited to the specific terms so selected and it is to be understoodthat each specific term includes all technical equivalents which operatein a similar manner to accomplish a similar purpose.

DETAILED DESCRIPTION

While the definitions of terminology, which is used in this document todescribe the preferred and alternative embodiments of the invention, aregenerally known to those skilled in the art, it is desirable to brieflyreview and expressly define a few of the terms which will be used.

A "transducer" is a device for converting useful energy in one form touseful energy to another form. For example, energy may be converted fromthe energy of mechanical motion to an electrical current or from thermalenergy to mechanical motion energy. Additionally, it is known that manytransducers which can be operated in one mode, can also be operated in areverse mode. For example, a device may be operated both as anelectrical motor to convert energy from electrical current to mechanicalrotation or reciprocation or it may be operated as a generator toconvert such mechanical motion to electrical current. Similarly, aStirling transducer may be operated either to convert thermal energy,flowing from a higher temperature to a lower temperature, to mechanicalmotion or it may be operated to utilize mechanical motion to pumpthermal energy, i.e. heat, from a lower temperature to a highertemperature. Therefore, the devices of this invention are referred to asStirling Cycle Transducers. Thus, it should be apparent that mostfeatures and embodiments of the present invention may be used both inthe engine mode, as well as in a cooling or refrigerator mode.

The term "bonded" is used in a general sense to describe separatelyidentifiable structures or layers which are mechanically joined,regardless of how they got that way. It includes not only two objects orlayers which are first separately constructed and then joined together,but also includes two structures or layers which are integrally formed,grown or deposited one in connection to the other.

FIG. 1 illustrates six electronic, integrated circuits, such asintegrated circuits 10 and 12, mounted upon a silicon substrate 14 andhaving interconnecting, conductor buses 16 formed on the substrate 14.Constructed beneath the integrated circuits are several Stirling coolersfor removing heat from the integrated circuits. Although the figureshows one Stirling cooler associated with each integrated circuit, suchone-to-one association is not necessarily required or implied. Each ofthese Stirling cycle coolers, such as the cooler 18, is a replication ofthe others and together they are shown arranged in a 2 by 3 array.

Spaced beneath the uppermost, silicon substrate 14 is a central supportplate 20 for supporting the centrally positioned regenerator of eachStirling cooler. Although a single support plate is indicated ascentrally located along the length of the regenerator, a multiplicity ofsupport plates might be employed and located anywhere along the lengthof the regenerator, including at each end of the regenerator such as endsupport plates 1151 and 1153 shown in FIG. 11. Beneath and parallel tothe central support plate 20 is a lowermost, second silicon substrate22.

The uppermost silicon substrate 14, the lowermost silicon substrate 22and the interposed portions of each Stirling cycle cooler form thepressure containing vessel of the Stirling cycle coolers. Each pressurecontaining vessel defines an enclosed workspace, including fluidpassages, and contains a compressible and expansible fluid, typically agas, all of which are needed for forming a Stirling cycle,thermomechanical transducer. In particular, the upper substrate 14 formsa heat accepting, fluid expansion end plate and the lower siliconsubstrate 22 forms a heat ejecting, fluid compression end plate for eachStirling cycle cooler. Interposed between these end plates is the heatregenerator which includes a perforate matrix in fluid communicationwith the end plates at its opposite ends.

These structures for one embodiment are illustrated in greater detail inFIGS. 2, 3, 4, and 5 and are described with reference to those figures.The end plates 14 and 22 may each comprise multiple, laminated layers.The structures of the end plates 14 and 22 are preferably formedutilizing planar processing technology of the type utilized inmanufacturing electronic integrated circuits and silicon sensors andactuators.

The heat accepting, fluid expansion end plate 14 defines an expansionspace 30, a back space 32 distal to the expansion space 30, and aflexible, expansion diaphragm 34 between the expansion space 30 and thebackspace 32. A heat accepting, heat exchanger, proximately bounds theexpansion space 30. In the preferred embodiment, the heat accepting heatexchanger is the heat accepting end plate 14, which bounds the workspace along interior wall 36 and includes the diaphragm 34, althoughalternatives may be used as described below.

Similarly, the heat ejecting, fluid compression end plate 22 defines acompression space 40, a backspace 42 distal to the compression space 40,and a flexible, compression diaphragm 44 forming a wall between thecompression space 40 and the backspace 42. A heat ejecting heatexchanger is formed by the end plate 22 which proximally bounds thecompression space 40 along the interior wall 46.

In order to control the frequency, phase and amplitude of thediaphragms, each is provided with an actuator which is a part of acontrol system. The expansion diaphragm 34 has an associated actuator 35and the compression diaphragm 44 has an associated actuator 45. Theactuators may, for example, include piezoelectric materials, which areelectrically connected to the remainder of the control system. Actuatorsare subsequently discussed in more detail below.

Interposed between the heat accepting end plate 14 and the heat ejectingend plate 22 is a heat regenerator 50, supported in place by the centralsupport plate 20. The heat regenerator 50 has a surrounding, typicallycylindrical, gas impervious wall 52 which contains the working gas andis sealingly connected to the end plate 14 and the end plate 22.Supported within the surrounding wall 52 is a perforate matrix 54 influid communication with both the expansion space 30 and the compressionspace 40. The perforate matrix illustrated in FIG. 4 comprises aplurality of spaced, planar walls 56 connected at their opposite sidesto the surrounding wall 52. Preferably the passages between these wallshave a cross-sectional aspect ratio greater than approximately 8.

The perforate matrix which forms the regenerator may have a variety ofconfigurations. It is a structure with passages which communicateaxially through the regenerator from the expansion space 30 to thecompression space 40. The regenerator must be capable of transportingthe gas axially and cyclically storing and releasing heat periodicallyso that the heat is pumped axially through the regenerator in incrementsfor each cycle. The passages may be formed in a regular pattern betweenlayers of material or may be a series of homogeneously distributed,interconnected pores in a foam-like material. Various alternativeembodiments of such regenerator structures are subsequently described.

It is desirable that the regenerator exhibit a very high internalsurface area to maximize the gas/regenerator interfacing surface andhave a high thermal capacity for the storage of heat. It is alsodesirable to minimize the thermal conductivity through the regeneratormaterial along the axial direction between the end plate 14 and the endplate 22. In a cooler, any heat conducted through the regeneratormaterial represents heat conducted back into the end being cooled and inan engine represents shunted heat which provides no mechanical workoutput. Consequently, a low thermal conductivity material, such asceramic or glass, is preferred, as well as a structure having a lowgeometrical, cross-section of mass.

It is a critically important feature of the present invention that thediameter or other corresponding lateral dimensions of the passagewaysthrough the perforate matrix of the regenerator be very small. It mustbe sufficiently small to provide a Wolmersley number below 5 at theoperating frequency of the Stirling thermomechanical transducer in orderto minimize the irreversibility losses associated with transporting thegas through those passages. Another critically important feature inorder to minimize the irreversibility losses associated withtransporting the working fluid through the aforesaid passages in thepresent invention is that the amplitude of diaphragm vibrations be verysmall. It must be sufficiently small to provide a maximum Mach numbersubstantially below 0.1 at the operating frequency of the Stirlingthermomechanical transducer.

These irreversibilities are a source of parasitic losses arising from ageneration of heat by friction between the working gas and the passagewalls. It results in viscous dissipation, from increased heat conductionby the working gas through the regenerator, and from reduced heattransfer in heat exchangers as the working frequency increases, thelatter two phenomena being peculiar to oscillatory flow as a result ofcertain phase relationships between the periodic variations in axialmass flow and the radial temperature gradient. In the present inventionthe lateral dimensions of these flow passages are on the order of 10-50microns and the amplitudes of diaphragm vibrations are on the order of10-100 microns in order to compensate for the increase in frequency. Theuse of such small passages and vibration amplitudes reduces theseparasitic losses. The reductions in these losses correspondinglyincrease the useful work which may be performed by the Stirling cycle,thermomechanical transducer in lifting heat or providing mechanicalwork.

A quantitative indication of the irreversibilities associated withoscillatory fluid flow in a passageway is given by the Wolmersleynumber. It is expressed by the equation:

    α=(2πf·a.sup.2 ρ/η).sup.1/2      I.

wherein:

α=Wolmersley Number

f=operating frequency

a=passageway diameter

ρ=working fluid density (which is a function of temperature andpressure)

η=working fluid dynamic viscosity (a function of temperature)

Another quantitative indication of irreversibilities associated withfluid flow in a passageway is given by the Mach number. It is expressedby the equation:

    Ma=2πf·x.sub.g /c.sub.0                        II.

wherein:

Ma=Mach number

x_(g) =the amplitude of oscillating axial gas displacement

c₀ =the velocity of sound in the working fluid

The axial oscillating working fluid displacement is related to theamplitude of diaphragm vibration by the equation:

    X.sub.g =X.sub.d ·A.sub.d /A.sub.x                III.

wherein:

X_(d) =the amplitude of diaphragm vibration

A_(d) =the area of the diaphragm vibration

A_(x) =the total cross-sectional area of all the passageways throughwhich the working fluid flows

The velocity of sound in the working fluid is well known as:

    c.sub.0 =(γRT).sup.1/2                               IV.

wherein:

γ=the ratio of specific heats at constant pressure and constant volume

=5/3 for helium

R=the specific gas constant for the working fluid

=2079 joules/kilogramkelvin for helium

T=absolute temperature in kelvin

Equations II, III, and IV can be combined to give:

    Ma=(2πf·x.sub.d ·A.sub.x /A.sub.d)/(γRT).sup.1/2 V.

In order to obtain the advantages of the present invention for operatingthe Stirling cycle transducer above 500 Hz, the passages must be madesufficiently narrow to provide a Wolmersley number less than 5 and theamplitude of diaphragm vibrations must be sufficiently small to providea maximum Mach number substantially less than 0.1 at the operatingfrequency. Preferably the Wolmersley number is made less than 1 and themaximum Mach number is made less than 0.01.

An example illustrating the parameters of a Stirling cycle transducerembodying the present invention illustrates the application of theseprinciples of the present invention. The example is directed to the useof a perforate heat exchanger of the type illustrated in FIGS. 8 and 9and described below.

The following example is for such a three-part microrefrigerator,charged with helium to 20 bars and operating at 1 kHz. Because there isa temperature gradient axially through the Stirling cycle coolerembodying the present invention, the temperature, and therefore theother parameters, are different at different axial positions. Thefollowing tables show one illustrative set of parameters wherein thewell known Reynolds number is related to the previously definedquantities by:

    Re=α.sup.2 x.sub.g /a

                  TABLE 1                                                         ______________________________________                                               Temp.   x.sub.g                                                                              a    ρ η                                        Location                                                                             (°K.)                                                                          (μm)                                                                              (μm)                                                                            (kg/m.sup.3)                                                                        (×10.sup.-6 Pa · s)                                                    Re   α                         ______________________________________                                        Cold Heat                                                                            100     200    10   9.8   9.5     13   0.81                            Exchanger                                                                     Mid-Re-                                                                              225     300    20   4.3   16.2    10   0.82                            generator                                                                     Warm   350     600    30   2.8   21.8    9.7  0.85                            Heat                                                                          Exchanger                                                                     ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                  Temp.   x.sub.d A.sub.d A.sub.x                                     Location  (°K.)                                                                          (μm) (×10.sup.-6 m.sup.2)                                                            (×10.sup.-6 m.sup.2)                                                            Ma                                  ______________________________________                                        Cold Heat 100     100      64     32      0.002                               Exchanger                                                                     Mid-Regenerator                                                                         225      100*    180*   32      0.004                               Warm Heat 350     100     290     32      0.005                               Exchanger                                                                     ______________________________________                                         *In the regenerator, averages of the two diaphragm displacements and area     are used, because the gas displacement is influenced by both diaphragms. 

Control System!

The theory of operation and various embodiments for controlling thephase and frequency of the expansion diaphragm 34 and the compressiondiaphragm 44 are taught by the prior art and therefore are not describedin detail. Such operation is described, for example, in theCooke-Yarborough U.S. Pat. No. 3,548,589. As the diaphragms vibrate theymove alternately into and away from their respective, associatedexpansion and compression spaces, thereby periodically varying theeffective volume of these spaces, alternately transporting gas into andout of the respective expansion and compression spaces.

FIG. 12, graphs 12A and 12B, illustrate the loci of the diaphragmdisplacement with respect to time at the opposite ends of theregenerator. Because these diaphragms face one another at opposite ends,a motion in opposite directions (for example, toward the center) has thesame effect in changing the volume of their associated spaces. Thus,volume changes with respect to time for the expansion and compressionspaces appear in FIGS. 12A and 12B as volumes V_(e) and V_(c). FIGS. 12Cand 12D are provided to illustrate the changes in the expansion spacevolume V_(e) and compression space volume V_(c) with respect to time. Asillustrated in these figures, the control system must control thediaphragms so that the expansion space volume leads the compressionspace volume by a nominal or approximate 90°. However, the phase lead isnot necessarily exactly 90, as known to those skilled in the art, andmay vary, for example, by as much as 100 or 200 for optimizingefficiency. For engines and coolers, the preferred volume phase leadsare greater than 90°. This approximate 90° phase lead must be maintainedfor both coolers and engines.

In order to maintain the proper phase, amplitude and frequency for theperiodic motion of the diaphragms, the control system includes anactuator, such as the actuators 35 and 45 referred to above. Preferablyeach actuator is an electromechanical transducer, mechanically linked toits associated diaphragm. Suitable transducers include magnetictransducers, such as taught by the above Cooke-Yarborough patent,piezoelectric transducers, magnetostrictive transducers, capacitivetransducers or other transducers which can drive the diaphragms.Electromechanical transducers are preferred because of the ease ofmaking connections to them and the ease of controlling the phase,amplitude and frequency of electrical drive signals.

As is known to those skilled in the art of alpha configuration, Stirlingcycle transducers, the working fluid does work on the piston associatedwith the expansion space, while the piston associated with thecompression space does work on the working fluid. Transducers arerequired for both pistons, to drive the piston associated with thecompression space, to dissipate power from the piston associated withthe expansion space, and to control the amplitude, frequency, and phaseof the motion of each piston. The term "actuators" is therefore used ina broad sense to include devices which can absorb or apply mechanicalenergy to or from the diaphragm and is not limited to those which onlygive mechanical work output. Therefore, it is the purpose of the controlsystem, including the actuators, to cause the diaphragms to vibrate withthe desired phase relation, amplitude and frequency.

A representative control system for maintaining the desired phase,amplitude and frequency of diaphragm vibrations is illustrated in FIG.13. Digital pulse train control signals S_(c), S_(EA) and S_(EB) at theresonant frequency f₀ of the microrefrigerator are derived by means of adigital counter 1301, decoder 1302, multiplexer 1303, and dividers 1304and 1305 from a single digital pulse train at a multiple of the resonantfrequency Nf₀. The phase lead of S_(EA) with respect to S_(c) is chosenby the Select inputs of the multiplexer 1303 to an arbitrary precisiondetermined by the width of the data path of digital counter 1301,decoder 1302, and multiplexer 1303. The fundamental sinusoidal componentS_(CO) of control signal S_(C) is extracted by the filter 1306 andapplied to the compression diaphragm motor-actuator 1307 shown in FIG.15. Control signal S_(EA) and its inverse S_(EB) are applied to thecontrol terminals A and A' and B and B', respectively, of a H-bridge oftransistors Q1-Q4 shown in FIG. 14. Excitation of this H-bridge bycontrol signals S_(EA) and S_(EB) actively rectifies the alternatingvoltage V_(EG) generated by expansion diaphragm generator-actuator 1308and appearing at its terminals 1308a and 1308b for charging the DC powersupply 1309 that back-biases the H-bridge. FIGS. 16 and 17 illustratethe directions of electrical current flow generated by the expansiondiaphragm generator-actuator 1308 through the H-bridge during alternatephases of the control signals S_(EA) and S_(EB). By this arrangement,electrical power flows from power supply 1309 into the compressiondiaphragm motor-actuator 1307 and out of the expansion diaphragmgenerator-actuator 1308 into power supply 1309. The difference betweenthe amount of electrical power that flows out of and into power supply1309 is the net power consumption of the micro-refrigerator. TransistorsQ1-Q4 are illustrated as bipolar transistors, but minor circuitmodifications would permit the same function to be performed by MOS orJFET transistors.

Integral End Plate Alternatives!

The use of silicon for forming the end plates 14 and 22 affords uniqueand major opportunities for the construction of a Stirling cycle,thermomechanical transducer. Because one important function of the endplates is to conduct heat, silicon provides advantageous results becauseit exhibits a high, thermal conductivity. Secondly, the choice ofsilicon permits the end components of the Stirling cycle transducer tobe formed utilizing prior art silicon planar processing techniques whichhave been used in the past for forming integrated, electronic circuitsand silicon transducers. Thirdly, the use of silicon permits suchintegrated electronic circuits to be joined to the expansion end plate14 with high reliability, despite the thermal cycling of the structure,because thermomechanical stress between the integrated circuits and thecold plate is eliminated due to their identical thermal coefficients ofexpansion. Fourthly, the use of silicon permits the electronic circuitsto be formed integrally with or within the expansion end plate 14 tomaximize the thermal conductivity for pumping heat away from theelectronic circuitry.

For example, and referring to FIG. 5, the end plate 14 may comprise twosilicon layers, an interior layer 60 and a cover layer 62. The diaphragm34 is formed by conventional, planar processing etching to form theexpansion space 30, and leaving the diaphragm 34. Backspace 32 isenclosed by bonding the cover layer 62 to the interior layer 60 so thatthe two silicon wafers are joined together in the plane by prior arttechniques, such as silicon fusion bonding or the use of an intermediaryfilm of glass or metal or an anodic bond. Similar or related techniquesare also used for forming the backspace 42 and the workspace 40 in thecompression end plate 22, which consists of interior layer 64 and coverlayers 66 and 68.

The areas of the expansion and compression diaphragms may be eitheridentical or different as indicated in FIG. 5.

Heat Exchanger Alternatives!

In embodiments of the invention heat is conducted through the expansionend plate 14 and transferred into the expansion space 30 and is alsotransferred from the compression space 40 into and then conductedthrough the compression end plate 22. As stated above, the interiorsurface walls of the portion of the work space at and within these endplates may serve as heat exchangers for transferring heat betweenworking gas in the respective expansion and compression spaces and theirassociated end plates.

FIG. 9, however, illustrates an alternative structure with a separateheat exchanger structure. The embodiment of FIG. 9 has an expansion endplate 914 and a compression end plate 922. It also has an expansiondiaphragm 934 and compression diaphragm 944, similar to thecorresponding parts in FIG. 5. However, a separate, silicon heatexchanger layer 980 is bonded to the silicon layer 960 and is interposedbetween this silicon layer 960 and the regenerator 950. This heatexchanger layer 980 is provided with a plurality of axial perforations982 communicating the expansion space 930 with the interior of theregenerator 950. Preferably these perforations are in the form of longslots separated by a series of parallel fins. A similar heat exchangerlayer 984 is bonded to the compression end plate layer 964 communicatingthe interior of the regenerator 950 with the compression space 942. Itis also provided with a plurality of transverse perforations 986.

Diaphragm Alternatives!

Planar processing techniques also permit the silicon diaphragm to beformed in configurations other than the flat sheet configuration, whichis illustrated in FIG. 5. FIG. 7, for example, illustrates an expansiondiaphragm 734 and compression diaphragm 744, having annularcorrugations. The expansion diaphragm 734 has annular corrugations 784and the compression diaphragm 744 has annular corrugations 786. Thecorrugations may be formed by etching notches into the diaphragm. Thesecorrugations make the diaphragm's pressure vs. displacementcharacteristic more nearly linear as is known in the art of constructingthe diaphragms used in low pressure sensors, such as barometers.

FIG. 9 illustrates that a boss region of increased mass, such ascircular boss 988, may be formed on the diaphragm 934 by planarprocessing techniques in order for the designer to control the mass ofthe vibrating diaphragm 934. This will assist the designer inconstructing the vibrating diaphragm so that it will vibrate in aresonant mode.

The ratio of diaphragm displacement to the force applied to thediaphragm can be increased by forming the diaphragm so that it isresonant at the operating frequency. As is well known from elementaryphysics, the natural frequency of oscillation of a mechanical system isa well known function of the oscillating mass and the spring constant ofan attached energy storing device, such as a spring, which alternatelyabsorbs and releases energy. The mass of a diaphragm in the embodimentof the invention as well as the resultant spring constant of all springsacting upon the diaphragm can be selected and designed to make thediaphragm resonant at the operating frequency. The mass can be selectedby the dimensions of the diaphragm and may be increased by providing aboss, as described above. The springs acting upon the diaphragm areprincipally the spring resulting from the elasticity of the silicondiaphragm itself, the gas spring resulting from the gas confined in thebackspace, and the gas spring resulting from the gas confined in thework space, which gas springs are adjacent to each diaphragm.Consequently, the gas spring, and particularly the backspace volume, maybe designed so that the diaphragm has a natural frequency ofoscillation, i.e. resonance, at the operating frequency. Alternatively,of course, the backspace volume may be so large that it has a negligiblespring constant and, as a consequence, the resonant frequency willprincipally be a function of the spring constants of the work space gassprings and of the silicon diaphragm itself and its mass. If theelasticity of the silicon is used as the principal spring constant, thebackspace 10 can entirely be eliminated, such as by venting it to thebackspace of an adjacent microrefrigerator unit in which the diaphragmsare vibrating out of phase by 180°.

The spring constant of the diaphragm can be reduced by making thediaphragm very thin. The spring constant of the diaphragm can also bemanipulated by the designer by forming layers of oxide or metal on it ordiffusing other materials into it, and, as a consequence, multilayerdiaphragms may also be utilized.

Diaphragm Actuators!

An electromechanical actuator, forming a part of the control system, ismechanically connected to each of the diaphragms. A broad variety ofelectromechanical actuators which may be used with the present inventionwill be apparent to those skilled in the art and may utilize, forexample, electrostatic, electromagnetic, piezoelectric ormagnetostrictive principles.

FIG. 5 illustrates a piezoelectric actuator 45 which is operated by anelectrical signal applied to its conductive leads 47 and 49.

FIG. 5A illustrates this actuator in more detail. It comprises a singleor a multiplicity of metal or conducting film electrodes 510, 512 and514, which are electrically connected together and connected to theconducting lead 49, and another single or a spaced multiplicity of metalor conducting film electrodes 516, 518 and 520, which are on theopposite side of a piezoelectric layer 522, such as zinc oxide, and areinterconnected together to conductive lead 47. This transducer may befabricated by forming an insulating oxide layer 524 upon the silicondiaphragm 526, then forming the metal or conducting film strips 510, 512and 514 by conventional techniques, then depositing a zinc oxide orother piezoelectric layer 522 upon the metal or conducting film stripsand the oxide layer 524, then again using conventional techniques todeposit the metal or conducting film strips 516, 518 and 520 upon thepiezoelectric layer 522. If desired, a further insulating layer of oxide528 may be deposited on top to provide a protective barrier andelectrical insulation.

In operation, the control system applies a periodic signal, at thefrequency of operation, to the leads 47 and 49, thereby inducing astress in the piezoelectric layer 522 and causing a resulting strain ofthe piezoelectric layer 522 and with it the motion of the diaphragm 526.

FIGS. 7 and 7A illustrate an actuator which utilizes electrostaticforces by forming capacitors 760 and 762 respectively in associationwith each of the diaphragms 734 and 744.

FIG. 7A illustrates the capacitor 762 in more detail. It has a capacitorplate 764 fabricated upon an oxide layer 765, which is deposited uponthe diaphragm 734 and connected to an electrical conducting lead 766,shown in FIG. 7A. It also has a second capacitor plate 767, similarlyfabricated upon an insulating oxide layer 768, deposited upon the endplate 769 and electrically connected to an electrically conducting lead770.

The application of a periodic electrical control signal, whichalternately charges the capacitor plates 764 and 767 with like chargesand opposite charges at the frequency of operation causes periodicforces of attraction for driving the diaphragm 734 in mechanicalvibration.

FIG. 9 illustrates yet another alternative actuator for driving thediaphragm 944. It comprises a pair of thin film coils 972 and 974 whichare formed as planar, conductive spirals respectively upon the end plate922 and the diaphragm 944. The planar coil 974 is connected toelectrically conducting leads 978 and 979 and the planar coil 972 isconnected to electrically conducting leads 991 and 993 for conduction ofelectrical control currents. While one electrical current may be DC, theother (or each if the first is not DC) is a periodic current at thefrequency of operation to provide a time varying magnetic field,alternately attracting and repelling the diaphragm toward and away fromthe stationary coil 972 to apply the mechanical stress to the diaphragm,causing it to vibrate at the appropriate phase and at the frequency ofoperation.

Regenerator Alternatives!

FIG. 7 illustrates a regenerator 750 having a reticulated foam 754contained within the outer circular, pressure containing vessel wall752. The reticulated foam 754 has continuously connected, open cellvoids or cavities along the entire length of the regenerator, throughwhich the working fluid flows. Such a regenerator may be constructedusing techniques currently used in the field of ceramic filters.

FIG. 8 and 9 illustrates an alternative regenerator formed by aplurality of parallel tubes 954, contained within the outer, circular,pressure containing vessel wall 952. The regenerator passages consist ofboth the passages through the center of each tube, as well as thepassages between the tubes.

FIG. 11 illustrates yet another alternative regenerator 1150 whichcomprises a plurality of spaced, concentric tubes 1154 contained withinand coaxial with the surrounding circular, pressure vessel wall 1152.

A regenerator may also be formed as a spirally wound, low thermalconductivity, solid film, such as a 3 mil thick glass foil, havingprojections deformed in the film so that when it is spirally rolled upand wound upon itself, the layers are spaced from each other to provideaxial passages.

Electronic Circuit Alternatives!

From the above description of the invention it can be seen that one ofthe major advantages of the present invention is that because a Stirlingcycle cooler embodying the present invention may be fabricated usingplanar processing, thin film and other techniques which are commonlyutilized in connection with the fabrication of integrated, electronicsemi-conductor circuits, coolers embodying the present invention canvery conveniently be fabricated so that they are physically andthermally intimately associated with the electronic circuits. Thecoolers may be fabricated as a part of or an extension of thefabrication of the electronic circuits. Further, these techniques allowhundreds of microrefrigerators to be manufactured simultaneously and inclose association with hundreds of electronic integrated circuits. Eachelectronic circuit is easily and conveniently fabricated so that it isclosely, thermally linked to the expansion end plate in order toefficiently pump heat from the electronic device so that the electronicdevice can be operated at a low, cryogenic temperature. The electronicdevices may be integrally fabricated directly into the expansion endplate or in films, discrete components or integrated circuits attachedto and thermally linked to the expansion end plate.

Furthermore, a refrigerator compartment, such as illustrated in FIG. 9,may be constructed in association with the end plate 914 by attaching asurrounding wall 926 for containing an electronic circuit 921 or otherobject to be cooled, and provided with a removable closure 928. Thisprovides an evacuated compartment 929, the contents of which arethermally insulated from the environment, except via the compression endplate 922, for containing and cooling an object, such as electroniccircuit 921. The term refrigerator is used to designate operation as athermal energy pump, pumping heat energy from a lower temperature at theexpansion end, to a higher temperature at the compression end. The termrefrigerator is not confined to application to a cooled compartment.

Engine Implementations!

Stirling cycle thermomechanical transducers embodying the presentinvention can also be designed and operated as an engine to providemechanical energy to a mechanical load. To accomplish this, a thermalenergy source is thermally linked to the expansion end and the controlsystem includes an energy conveying link from the compression enddiaphragm to a load. The load must be a part of the control systembecause the complex load impedance is part of the dynamic systemdetermining the magnitude, phase and frequency of diaphragm vibrationsdescribed above.

FIG. 11 illustrates a compression end diaphragm 1110, connected to amechanical load 1112 through a connecting rod 1114, operating as anenergy conveying link. The load 1112 may be, for example, an electricalgenerator, duplex microrefrigerator or a fluid pump.

The coils in FIG. 9 may be utilized in an embodiment of the inventionoperated as an engine for the purpose of generating electrical power.

Thermal energy may be applied to expansion end plate 1116 in any of theconventional manners by which thermal energy is applied to Stirlingcycle engines including incident solar radiation, combustion,radio-isotope radiation or industrial waste heat.

The present invention affords the opportunity for fabrication of microengines which can be utilized to efficiently convert the solar or otherthermal energy to electrical energy or mechanical energy. Becausecurrently available photocells convert solar radiation energy directlyto electrical energy by an opto-electronic process called thephotovoltaic effect, they suffer several disadvantages in comparisonwith the present invention. First, the efficiency of thethermomechanical conversion of energy by Stirling engines is superior tothe opto-electronic conversion efficiency of photovoltaic solar cells..In addition, more solar energy is available for conversion by a Stirlingengine than by a photocell: while only approximately 25 k of the energyin the solar spectrum is within the band of wavelengths that excites aphotovoltaic effect, the energy of the entire solar spectrum can beconverted to heat for driving the Stirling cycle. Furthermore, withphotocells, the amount of energy available for conversion can not beincreased by concentrating solar radiation, because that would raise thetemperature of the photocell far beyond the very modest temperature atwhich the necessary electronic properties of semiconductors are lost. Incontrast, solar radiation can be concentrated onto the silicon heatacceptor of a Stirling micro-engine, because it employs thethermomechanical and not the electronic properties of silicon.

Fabrication Methods!

Embodiments of the present invention are preferably fabricated byadapting current planar integrated circuit fabrication processes to theproduction of these embodiments. These processes includemicrolithography, various oxidation, deposition, doping, etching andother techniques employed in the integrated circuit and silicon sensorand actuator industries. The electrical, integrated circuit chip, whichis to be cooled to subambient temperatures, can be manufactured as apart of the same wafer as a cryocooler embodying the present invention.The silicon surface of the cryocooler can be the substrate within orupon which the electronic circuit is constructed.

The structures of the present invention lend themselves well also to thefabrication of large numbers of multiple replications upon a singlewafer and subsequent separation of the wafer into either individualembodiments or groups of multiple embodiments. For example, embodimentsmay be constructed by forming a cooperating diaphragm, back space,expansion space and heat exchanger for each of a plurality of spacedapart, heat accepting, fluid expansion ends of a plurality of Stirlingtransducer pressure vessels. This can be done by etching away selectedportions of silicon wafers and then aligning and joining the waferstogether in-the-plane as an integral expansion end plate. Similarly, acooperating diaphragm, back space, compression space and heat exchanger,with each of a plurality of spaced apart, heat ejecting, fluidcompression ends of a plurality of Stirling transducer pressure vesselsmay also be formed by etching away selected portions of silicon wafersand then aligning and joining the wafers together in-the-plane as anintegral compression end plate.

A plurality of heat regenerators may be formed and joined together as anintegral regenerator plate, each regenerator having fluid openings onopposite sides of the regenerator plate. The openings are spaced apartfor registration with the end plates. The regenerator plate is theninterposed between, aligned with and bonded to the expansion end plateand the compression end plate, by a technique, such as Mallory bondingdescribed in U.S. Pat. No. 3,397,278, to form a unitary structurecomprising a plurality of Stirling cycle thermomechanical transducers.This unitary structure may then be separated into individual transducersor separated into a plurality of arrays of multiple transducers.

One manner for forming the integral regenerator plate is to form aplurality of individual, substantially identical regenerators and thenbond the regenerators into holes in one or more of the previouslymentioned support plates, the holes being arranged in a parallel,laterally spaced array registered with the end plates. The array ofindividual regenerators is then mechanically connected together to formthe regenerator plate. The regenerator plate is then joined between thetwo end plates.

Conclusion!

From the above it is apparent that in embodiments of the presentinvention the entropy generating processes in oscillatory flow aregreatly reduced which allows the embodiment to operate at much higherfrequencies than previously thought possible for Stirling machines andthus allowing a tiny machine to have a practical thermal pumpingcapacity and a very desirable specific capacity. While prior artStirling cycle machines have probably operated at the conventional lowerfrequencies with a Wolmersley number below 5, the significance of therelationship between the passage size and the Wolmersley number andbetween displacement amplitude and Mach number have never beenassociated with an opportunity to build small, high frequency machines.

The broad concept of the invention is the combination of passagesexhibiting a characteristic Wolmersley number below 5 and Mach numbersbelow 0.1 combined with a frequency of operation above 500 Hz.

Because of the dramatic increase in the thermal conductivity of siliconas temperatures decline, silicon makes an exceptionally desirablematerial for low temperature heat exchangers and heat conductingcomponents. At the same time, silicon is an ideal substrate material forthe attachment or fabrication of silicon chips since there is nodifference in thermal coefficient of expansion between the substrate andthe chip, which, if there were, would otherwise tend to cause detachmentor separation under extreme temperature excursions that occur in acryocooler. Since silicon is the most common material in whichelectronic circuits are fabricated, silicon offers the possibility offabricating circuits into the structure of the microrefrigerator's heatexchanger itself. The circuits fabricated into the cooled end heatexchanger might be the circuits that the machine is designed to cool, orthey might be circuits that control the operation of the machine.

The small size and high frequency of this machine allows the machine tooperate in near-isothermal conditions, unlike the less energy efficientadiabatic conditions in previous, larger, higher frequency machines.

Improvements

While the definitions of terminology, which is used in this document todescribe the preferred and alternative embodiments of the invention, aregenerally known to those skilled in the art, it is desirable to brieflyreview and expressly define additional terms which will be used.

In the context of thermomechanical transducers, the term "piston" isused to refer to a reciprocating component across which a pressuredifference is supported, whereas the term "displacer" is used to referto a reciprocating component across which a temperature difference issupported.

Thermomechanical transducers incorporating a regenerator are referred toas regenerative transducers. One type of regenerative transducer is aStirling cycle transducer. It is well known that the Stirlingthermodynamic cycle may be implemented by transducers with variousconfigurations of internal components. In the alpha configurationStirling cycle transducer, the two internal reciprocating componentsboth contribute substantially to the compression and expansion as wellas the displacement of the working fluid. Therefore, these componentsare known as the compression piston and the expansion piston Thefrequency, amplitude, and phase of the motions of both these pistons iscontrolled by either a mechanical linkage or by electromechanicalactuators electrically linked to a control circuit apparatus.

By contrast, a beta configuration Stirling cycle transducer employs onepiston and one displacer. The beta configuration is especially wellsuited for use in free-piston Stirling cycle transducers in which thefrequency, amplitude, and phase of the piston and displacer motions arecontrolled by the arrangement of the masses of these components, by thesprings connected to them, and by the pressure forces acting upon them.As a result, one advantage of the beta configuration over the alphaconfiguration is that the beta configuration eliminates at least oneelectromechanical transducer from the control system.

It is known that a beta configuration Stirling cycle transducer operatedto convert thermal energy to the mechanical motion of a piston may beconnected to another beta configuration Stirling cycle transduceroperated to utilize that motion to pump thermal energy from a lowertemperature to a higher temperature. Devices of this type are referredto as duplex Stirling cycle transducers, and they require noelectromechanical transducers for energy conversion, although they mayemploy an electromechanical transducer in an auxiliary starting system.

Another type of regenerative transducer is a Vuilleumier heat pump. Likea duplex Stirling cycle transducer, a Vuilleumier heat pump may also beoperated to convert thermal energy, flowing from a high temperature toan intermediate temperature, to pump thermal energy from a lowtemperature to an intermediate temperature. However, the Vuilleumierheat pump achieves this effect by thermal compression without involvingthe mechanical motion of a compression piston. Conceptually, therefore,a Vuilleumier heat pump resembles a duplex Stirling cycle transducerwithout a mechanical compression piston.

In the field of microelectronics, the term "integrated" refers toelectrical circuits the component parts of which are constructed asportions of a single mechanical object and which, therefore, do not haveto be mechanically connected together such as by soldering after theirfabrication in order to perform their function as an electrical circuit.These integrated circuits are fabricated in large numbers simultaneouslyin a unitary silicon wafer and subsequently separated into individuallyfunctional circuit chips. In the field of microsensors andmicroactuators, the term "integrated" refers to transducers thecomponent parts of which are also constructed as portions of a singlemechanical object and which, therefore, which do not have to bemechanically connected together after their fabrication such as bywelding or bolting in order to perform their function as a transducer.These integrated transducers are fabricated in large numberssimultaneously in a unitary silicon wafer and subsequently separatedinto individually functional transducer chips. In the field ofmicrosensors and microactuators, the bonding of wafers togetherin-the-plane into a laminated structure before the wafers are separatedinto chips is regarded as part of the integrated transducer fabricationprocess and not as part of a subsequent process for connecting componentparts of a transducer together. Thus, the term "integrated pressuresensor" is used to refer to a chip of glass and silicon which serves thefunction of a pressure sensor, the component parts of which werefabricated simultaneously with those of many other such transducers inwafers of silicon and glass, which wafers were bonded together beforebeing divided into separate functional transducers.

Stirling Cooler!

FIG. 18 illustrates two electronic devices 1801 and 1802 mounted upon aheat absorbing silicon plate 1803 and having interconnecting, conductors104 formed on the plate 1803. Constructed beneath the electronic devicesare two free-piston Stirling coolers for removing heat from and loweringthe temperatures of the electronic devices. Although the figure showsone Stirling cooler associated with each integrated circuit, suchone-to-one association is not necessarily required or implied. Each ofthese coolers, such as the cooler 1805, is a replication of the otherand together they are shown arranged in a 1 by 2 array, although theymay be fabricated in larger two-dimensional arrays.

Spaced beneath and parallel with the heat absorbing silicon plate 1803is a heat ejecting silicon plate 1806. Interposed between these platesis the regenerative displacer 1807. The regenerative displacer 1807includes a heat regenerator 1810 with a perforate matrix in fluidcommunication with the heat absorbing plate 1803 and heat ejecting plate1806 at its opposite ends.

The heat absorbing silicon plate 1803, the heat ejecting silicon plate1806 and the interposed regenerative displacer 1807 form the pressurecontaining vessel of the Stirling cycle coolers. Each pressurecontaining vessel defines an enclosed workspace, including fluidpassages, and containing a compressible and expansible fluid, typicallya gas, all of which are needed for forming a Stirling cycle,thermomechanical transducer.

The plates 1803 and 1806 may each comprise multiple, laminated layers.The structures of the silicon plates 1803 and 1806 and of theregenerative displacer 1807 are preferably formed utilizing planarprocessing technology of the type utilized in manufacturing electronicintegrated circuits and silicon sensors and actuators.

In the preferred embodiment, the heat accepting heat exchanger is theinterior wall 1813 which, together with the displacer diaphragm 1808,bounds the expansion space 1814, although alternatives may be used asdescribed below. Similarly, the heat ejecting heat exchanger is thepiston diaphragm 1811 which, together with the displacer diaphragm 1809defines a compression space 1815, although alternatives may be used asdescribed below.

The heat ejecting plate also includes a piston 1816 comprised of a boss1817 and a flexible diaphragm 1811. To control the frequency andamplitude of the piston, the piston is connected via an energy conveyinglink 1818 to a motive power means 1819. Examples of such motive powermeans include electromechanical actuators, such as electromagnetic,electrostatic, and magnetostrictive transducers, as well asthermomechanical transducers such as Stirling, Otto, and Diesel cycleengines. Stirling cycle engines are discussed in more detail below.

Interposed between the heat accepting plate 1803 and the heat ejectingplate 1806 is a regenerative displacer 1807 comprised of a regenerator1810 supported in place by flexible displacer diaphragms 1808 and 1809and a plurality of walls 1820 for connecting the regenerative displacerto the spaced heat absorbing and the heat ejecting plates 1803 and 1806.In the preferred embodiment, the regenerator 1810 has a surrounding wall1821 which substantially contains the working fluid. Within thesurrounding wall 1821 is a perforate matrix 1822 in fluid communicationwith the expansion space 1814 and the compression space 1815. In thepreferred embodiment, the perforate matrix comprises a plurality ofspaced, planar walls 1823 connected at their opposite sides to thesurrounding wall 1821, although alternative embodiments may be used.Preferably the passages between these walls have an aspect ratio greaterthan approximately 8.

To reduce the internal conduction of heat between the heat ejectingplate 1806 and the heat accepting plate 1803, it is desirable that thethermal conductivity through the regenerative displacer material alongthe axial direction between the heat accepting plate 1803 and the heatejecting plate 1806 be low and that the cross sectional area of thismaterial be small. By contrast, it is desirable for the thermalconductivity of the heat ejecting plate 1806 and of the heat acceptingplate to be high in the all directions to reduce temperature dropsbetween heat sources and heat sinks on the one hand and the workingfluid on the other. In practice, a thermal conductivity of approximately1 W/mK is low enough for regenerative displacer material, and a thermalconductivity of 10 W/mK is high enough for the material of heataccepting and heat ejecting plates. Silicon dioxide has a low thermalconductivity material for regenerative displacer purposes, having athermal conductivity of approximately 1.4 W/mK. Silicon is a highthermal conductivity material for heat accepting and heat ejecting platepurposes, having thermal conductivities ranging from 25 to 1000 W/mKbetween temperatures of 1000 K and 50 K. Another high thermalconductivity material with even higher thermal conductivities in thistemperature range is silicon carbide.

Furthermore, it is desirable for the three functional parts of theregenerative displacer (the regenerator, the displacer, and theconnecting walls) to be fabricated simultaneously as portions of asingle mechanically integrated structure by industrial processes similarto those employed to fabricate the heat accepting and heat ejectingplates of the transducer as mechanically integrated structures. In theimproved embodiment of the invention, all these objectives are achievedby anisotropically etching silicon into the three dimensional shape ofthe unit structure of the regenerative displacer, by oxidizing this unitstructure into silicon dioxide which has a thermal conductivity an orderof magnitude or more lower than that of silicon, and by bonding theseunit structures together into a mechanically integrated regenerativedisplacer component. These structures for one embodiment of aregenerative displacer are illustrated in greater detail in FIGS. 22 and23 and are described with reference to those figures.

Control System!

In a beta configuration free-piston Stirling cycle transducer, themasses of the piston and displacer, the springs acting upon the pistonand displacer, the areas of various portions of the displacer, and thedamping of the motions of the displacer and piston comprise a mechanicalcontrol system that controls the proper phase, amplitude, and frequencyfor the periodic motion of the piston and displacer. The theory ofoperation and various embodiments for controlling the frequency, phase,and amplitude of the piston and displacer of beta configuration Stirlingcycle transducers are taught by the prior art and therefore are notdescribed in detail. Such operation of a beta configuration Stirlingtransducer operating as an engine, for example, is described byBerchowitz and Redlich ("Linear dynamics of free-piston Stirlingengines", Proc Instn Mech Engrs, Vol 199, No A3, pp. 203-213), whereasoperation of a beta configuration Stirling transducer operating as acooler is described by Berchowitz ("Free-piston Stirling coolers", Proc.International Refrigeration Conference Energy Efficiency and NewRefrigerants, Purdue University, Jul. 14-17, 1992).

As the pressure of the working fluid alternates in a beta configurationStirling transducer, a force equal to the product of the amplitude ofthe alternating pressure and the area of the piston exposed to theworking fluid acts on the piston. The displacer is constructed such thatthe portion of its area Ad1 exposed to the working fluid in the heatabsorbing region of the transducer is greater than the portion of itsarea Ad2 exposed to the working fluid in the heat ejecting region. Athird portion of its area Ad3=Ad1=Ad2 is exposed to a differentreference pressure which varies during a cycle much less than does theworking fluid pressure. As a result of this arrangement, an alternatingforce also acts on the displacer in such a way that the common frequencyof the alternating motions of the piston and displacer, the ratio of theamplitudes of these motions, and the phase angle between these motionsare determined by such factors as Ad3, the spring constant acting on thedisplacer, the Q of the displacer (i.e., the ratio of the storeddisplacer energy and the energy dissipated by the displacer each cycle),the undamped natural frequencies of the displacer and piston, and thedamping of the displacer motion by various dissipative processes. Thespring constant is implemented by exploiting the spring properties ofthe diaphragm and of the working fluid working as a gas spring.

A representative control system for maintaining the desired phase,amplitude, and frequency of piston and displacer diaphragm vibrations isillustrated in FIG. 18. The mass of the regenerative displacer 1807 andthe spring constants of the displacer diaphragms 1808 and 1809 determineits undamped natural frequency. The dissipation of energy by the viscousflow of the working fluid through the regenerator 1810 determines thedamping of the displacer motion. Together, this mass, these springconstants, and this damping determine the Q of the displacer. The areaof the displacer diaphragm 1808 is greater than the area of thedisplacer diaphragm 1809, because the diameter between the fixed edgesof diaphragm 1808 is larger than the diameter between the fixed edges ofdiaphragm 1809. The alternating differential pressure between thepressure of the working fluid in the work space and the pressure of thefluid between the displacer diaphragms acts over the difference in theareas of the displacer diaphragms to produce the net force acting on theregenerative displacer. The mass of the piston 1810 and the springconstant of the piston diaphragm 1811 and of the working fluid in theenclosed workspace determine its undamped natural frequency.

In some embodiments, it is convenient for the surrounding wall of theregenerator or for one or more of the displacer diaphragms to containone or more perforations to enable the working fluid to fill thechambers between the displacer diaphragms, but these perforations mustbe so small as not to substantially disturb the oscillating flow andpressure of working fluid within the work space during the operation ofthe transducer. In such embodiments, the interconnecting walls 1820 mustcontain the working fluid. In alternative embodiments in which thechambers between the displacer diaphragms are not filled with fluid fromthe work space, the reference pressure in these chambers may beestablished by means of small or large perforations in theinterconnecting walls 1820.

Stirling Engine!

Stirling cycle thermomechanical transducers embodying the presentinvention can also be designed and operated as an engine to providemechanical energy to a mechanical load. To accomplish this, a thermalenergy source is thermally linked to the heat absorbing region and themechanical load is mechanically linked to the piston. The energydissipating and energy storing effects of the mechanical load are partof the mechanical control system determining the amplitude, phase, andfrequency of the motions of the piston and displacer.

Referring to FIG. 19, thermal energy may be applied to the heataccepting plate 1902 in any of the conventional manners by which thermalenergy is applied to Stirling cycle engines, including incident solarradiation, combustion, radio-isotope radiation or industrial waste heat.The heat source illustrated in FIG. 19 is an electrical resistor 1901thermally linked to the heat accepting silicon plate 1902. Constructedabove the heat source is a free-piston Stirling cycle engine 1903 forpumping fluid through an open or closed loop of passages. Although thefigure shows one Stirling engine associated with one heat source, suchone-to-one association is not necessarily required or implied.

Spaced beneath and parallel with the heat accepting silicon plate 1902is a heat ejecting silicon plate 1904. Interposed between these platesis the regenerative displacer 1905. The regenerative displacer 1905includes a heat regenerator 1906 with a perforate matrix in fluidcommunication with the heat absorbing plate 1902 and heat ejecting plate1904 at its opposite ends.

The heat absorbing silicon plate 1902, the heat ejecting silicon plate1904 and the interposed regenerative displacer 1905 form the pressurecontaining vessel of the Stirling cycle engine. The pressure containingvessel defines an enclosed workspace, including fluid passages, andcontaining a compressible and expansible fluid, typically a gas, all ofwhich are needed for forming a Stirling cycle, thermomechanicaltransducer.

In the embodiment shown in FIG. 19, the heat accepting plate 1902includes a heat source substrate layer 1938 and a heat accepting heatexchanger layer 1907 comprised of an extended surface 1908. Preferably,the extended surface 1908 comprises a plurality of spaced, planar walls1939 connected at their opposite sides to the heat exchanger layer,although alternative embodiments may be used. Preferably the passagesbetween these walls have an aspect ratio greater than approximately 8.

In the embodiment shown in FIG. 19, the heat ejecting plate 1904includes a heat ejecting heat exchanger layer 1909, two piston layers1910a and 1910b, and an integral valve head 1911. Like the heataccepting heat exchanger layer 1907, the heat ejecting heat exchangerlayer 1909 has an extended surface 1912, preferably comprised of aplurality of spaced, planar walls 1913 with the passageways between thewalls having an aspect ratio greater than approximately 8. This pistonlayers 1910a and 1910b comprise a boss 1914 and two flexible diaphragms1915 and 1916, although other numbers of layers and diaphragms may alsobe used. The piston diaphragm 1915, together with the heat ejecting heatexchanger layer 1909, bounds the compression space 1917. In theembodiment shown in FIG. 2, the piston layers 1910a and 1910b are joinedby means of a metal or glass film. That technique can also be usedbetween other layers.

Interposed between the heat accepting plate 1902 and the heat ejectingplate 1904 is a regenerative displacer 1905 comprised of a regenerator1903 supported in place by displacer diaphragms 1918, 1919, and 1920 anda plurality of walls 1921 for connecting the regenerative displacer 1905to the spaced heat absorbing and the heat ejecting plates 1902 and 1904.The regenerator 1903 has a surrounding 1922 wall which contains theworking fluid in the preferred embodiment. Within the surrounding wall1922 is a perforate matrix 1923 in fluid communication with theexpansion space 1924 and the compression space 1917. The expansion space1924 is bounded by the heat accepting heat exchanger layer 1907 and thedisplacer diaphragm 1920. In the preferred embodiment, the perforatematrix 1923 comprises a plurality of spaced, planar walls 1925 connectedat their opposite sides to the surrounding wall 1922, althoughalternative embodiments may be used. Preferably the passages betweenthese walls have an aspect ratio greater than approximately 8.

In the embodiment shown in FIG. 19, the fluid valve head is an energyconveying link mechanically connecting the piston via the pumped fluidto a mechanical load consisting of the fluid flow through the attachedopen or closed loop of passages. Integral silicon valves are well knownin the silicon actuator industry. The integral valve head 1911 shown inFIG. 19 is comprised of three bonded silicon layers which include inletand outlet ports, internal passages, and suction and discharge valvecomponents. In operation, as the piston boss 1914 moves away from valvehead 1911, the pressure in the fluid pump compression space 1926declines drawing the suction valve boss 1927 suspended on the flexiblesuction valve diaphragm 1928 and away from the suction valve seat 1929,allowing fluid to flow through the inlet port 1930, past the suctionvalve seat, and through other internal passage 1931 and 1932 into thefluid pump compression space 1926. At the same time, declining pressurein the fluid compression space 1926 draws the discharge valve boss 1933suspended on the discharge valve diaphragm 1934 against the dischargevalve seat thereby preventing fluid from flowing from the discharge port1935 into the fluid pump compression space.

As the piston boss 1914 moves toward from valve head 1911, the pressurein the fluid pump compression space 1926 rises pushing the suction valveboss 1927 against the suction valve seat 1929, preventing fluid in thefluid pump compression space from flowing out the inlet port 1930. Atthe same time, rising pressure in the fluid compression space 1926pushes the discharge valve boss 1933 away from the discharge valve seatthereby allowing fluid to flow from the fluid pump compression spacepast the discharge valve seat, through the internal passages 1935 and1936 and out the discharge port 1937.

Duplex Stirling Cooler!

FIG. 20 illustrates one embodiment of a duplex free-piston Stirlingcycle transducer in which a common heat ejecting silicon plate 2002includes two heat ejecting heat exchanger layers 2009 and 2010 and twopiston layers 2011 and 2012. Thus, a beta configuration Stirling engine2001 is mechanically linked via a common piston 2013 to a betaconfiguration Stirling cooler 2003 to comprise a duplex Stirling cycletransducer without any electromechanical transducers being needed forenergy conversion. Establishment of a sufficient temperature differencebetween the working fluid in the engine expansion space 2014 and theengine compression space 2015 causes the regenerative displacer 2006 ofthe Stirling engine 2001 and the common piston 2013 to reciprocate. Asdescribed above, reciprocation of the common piston 2013 causes theregenerative displacer 2016 of the Stirling cycle cooler 2003 toreciprocate so that heat is absorbed by the heat accepting silicon plate2017 causing its temperature to decline so that the temperature of theworking fluid in the expansion space 2018 of the Stirling cycle cooleris lower than the temperature of the working fluid in the compressionspace 2019 of the Stirling cycle cooler.

The heat source 2004 illustrated in axial section in FIG. 20 is aninternal electrical resistor thermally linked to the edges of theregenerator fins 2005 of the Stirling cycle engine's regenerativedisplacer 2006. FIG. 20A illustrates in plan view through cross sectionA-A' the electrical resistor 2004 as a ribbon of electrically resistivematerial distributed between electrical leads 2007 and 2008 along acontinuous, meandering path on the edges of the regenerator fins 2005.Other parallel as well as series electrical paths of multiple materialsof various electrical conductivities could also be employed. Materialproperties suitable for the electrical conductors in the hot end of theStirling cycle engine include high melting point, low thermal diffusioncoefficient, and low chemical reactivity. Examples of such materialsinclude refractory metals (such as molybdenum, titanium, tungsten,tantalum, and zirconium), the nitrides, aluminides, and suicides ofthese metals, and noble metals (such as platinum, rhodium, and niobium).

One means for passing an electrical conductor through the pressurevessel of the Stirling cycle engine is shown in FIG. 20B. In FIG. 20B,the electrical leads 2007 and 2008 are hermetically sealed between thinfilms of electrically insulating material (such as silicon dioxide,silicon nitride, or high melting point glass). One of these insulatingfilms 2011 is attached to the hot silicon end plate 2009 while anotherinsulating film 2012 is attached to the hot end of the regenerativedisplacer plate 2020. The insulating film 2013 fills the space betweenand around the electrical leads 2007 and 2008 to planarize the interfacebetween films 2011 and 2012 to facilitate the establishment of anhermetic seal. We currently believe that a combination of platinum onniobium is preferred, because platinum wires can be used to makeelectrical connections to the platinum film and because the niobium filmcan serve as a barrier to diffusion at high temperature due to its lowthermal diffusion coefficient.

To facilitate heat transfer from the heat source 2004 to the workingfluid, the hot end of the regenerator may not be oxidized, in which casethe unoxidized portion of the regenerator would function as anapproximately isothermal heat accepting heat exchanger transferring heatunidirectionally from the unoxidized portion of the fins 2005 to thegas, instead of as a regenerator with a strong axial temperaturegradient transferring heat bidirectionally depending on the direction offlow of the working fluid.

Vuilleumier Heat Pump!

The principles of operation of a Vuilleumier heat pump are well known tothose skilled in the art of thermomechanical machines. Like the duplexStirling cycle transducer, the Vuilleumier heat pump requires noelectromechanical transducer for energy conversion. An advantage of theVuilleumier heat pump over the duplex Stirling cycle transducer in someapplications is that the Vuilleumier heat pump produces less vibrationdue to its elimination of the relatively massive piston of the duplexStirling cycle transducer. A related disadvantage of the Vuilleumierheat pump compared to the duplex Stirling cycle transducer for otherapplications is a smaller specific capacity, since the Vuilleumier heatpump does not benefit from the volumetric compression and expansionprovided by the piston of the duplex Stirling cycle transducer.

As with Stirling cycle transducers, the frequency, amplitudes and phaseof the motions of the reciprocating components (two displacers in thiscase) in Vuilleumier heat pumps may be controlled either by mechanicallinkages and electromechanical transducers or by the masses of thereciprocating components themselves, the springs attached to them, andby the damping and pressure forces arising inside the machine. Theoperation of the former (kinematic) type of Vuilleumier heat pump hasbeen described by Walker ("Vuilleumier Cryocoolers," in Cryocoolers,Part 1: Fundamentals, Plenum Press, New York, 1983, pp. 185-236,especially pp. 212-220). The operation of the latter (free-piston) typeof Vuilleumier heat pump has been described by Schultz and Thomas ("Alinear model of a free-piston Vuilleumier machine compared toexperimental results of a prototype," 27th Intersociety EnergyConversion Conference Proceedings, IECEC 1992, San Diego, Calif., Aug.3-7, 1992, Volume 5, pp. 5.75-5.80.) Schultz and Thomas show thatfree-piston Vuilleumier heat pumps may achieve stable oscillationwithout a spring between the casing and the cold displacer, and withouta spring between the two displacers, but not without a spring betweenthe hot displacer and the casing unless the casing motion is largecompared to that of the displacers.

FIG. 21 illustrates an embodiment of a free-piston Vuilleumier heat pumpcomprised of a hot heat accepting silicon plate 2101, a hot regenerativedisplacer plate 2102, a warm heat rejecting silicon plate 2103, a coldregenerative displacer plate 2104, and a cold heat accepting siliconplate 2105. The hot regenerative displacer includes flexible displacerdiaphragms 2106 and 2107 which function as springs linking the mass ofthe hot regenerative displacer to the casing 2108. The casing 2108includes all the heat accepting and heat rejecting plates as well as thewalls of the regenerative displacers that connect these plates together.Similarly, the cold regenerative displacer includes flexible displacerdiaphragms 2109 and 2110 which function as springs linking the mass ofthe cold regenerative displacer to the casing 2108.

As in the Stirling cycle transducers, it is desirable for the thermalconductivity of the regenerative displacers in the axial direction to beminimized and for thermal cross section of this material to beminimized. Also, as in the Stirling cycle transducers, the areas of thedisplacer diaphragms 2106 and 2110 nearest the heat accepting regionsare larger than the areas of the displacer diaphragms 2107 and 2109nearest the heat ejecting regions in order to provide the required areadifferentials to create the alternating forces that drive the displacermotions. Also as in the Stirling cycle transducers described above, thereference pressure involved in the control of each regenerativedisplacer is the pressure of the fluid in the chambers 2111 and 2112between the displacer diaphragms

Regenenerative Displacer Details!

It is well known that the silicon crystal may be cut into wafers suchthat a normal vector to the plane of these wafers is in a selectedcrystallographic direction of the silicon crystal. Thesecrystallographic directions are denoted by their so-called Millerindices which are in the form <XYZ>, wherein X, Y, and Z represent thelengths of vectors in a Cartesian coordinate system aligned with thecrystallographic structure. It is also well known that the siliconcrystal is anisotropic with respect to many of its properties. That is,these properties have different values in different crystallographicdirections. For microelectronic circuit purposes, for example, waferswith their plane normal to one of the <100> family of crystallographicdirections are usually preferred due to the values of certain electronicproperties of the crystal in that direction. Amongst the anisotropicproperties of silicon is he solubility of the silicon crystal in certainchemicals. For example, the silicon crystal etches much faster in thefamilies of <100> and <110> directions in acqueous solutions ofpotassium hydroxide than in the family of <111> directions. Furthermore,silicon etches much faster in <100> and <110> directions than doessilicon dioxide. This chemical anisotropy and selectivity is widelyemployed to form complicated three-dimensional structures for integratedsensors and actuators.

It is desirable for the regenerator of a microminiature regenerativetransducer to be both highly anisotropic in structure (preferably for itto be comprised of a multiplicity of spaced, parallel walls) and for thematerial of these walls to be low in thermal conductivity. However, thematerials that lend themselves to the formation of highly anisotropicstructures are crystalline, and crystalline materials have relativelyhigh thermal conductivities. Conversely, because materials with lowthermal conductivities are amorphous, they are difficult to form intohighly anisotropic structures. The present invention overcomes thisdifficulty by exploiting the chemical properties of silicon, so that thehighly anisotropic structure of the regenerative displacer is formed inan anisotropic silicon wafer, and then the resulting structure isoxidized into amorphous silicon dioxide which has a much lower thermalconductivity.

FIG. 22A illustrates one side of a <110> silicon wafer 2201. Thecrystallographic orientation of a <110> wafer may be uniquely specifiedby the location of certain flats 2202 and 2203 polished onto the edge ofthe wafer. In the embodiment shown in FIG. 22A, the orientation isindicated by flats aligned with <111> planes located 70.53 degreesapart.

FIG. 22B illustrates an expanded plan view of an irregular but symmetrichexagonal cavity 2204 etched through a hole in a silicon dioxide filmcovering a <110> silicon wafer such as the one shown in FIG. 22A. Aftersufficient etching time has passed for the etching process to terminateat <111> planes, the cavity 2204 shown in FIG. 22B would result from anyand every hole that had edges tangent to the six lines (AB, BC, CD, DE,EF, and FA) where the six <111> planes that constitute the sides of thecavity 2204 intersect the surface of the wafer. Of the six <111> planes,four planes 2205, 2206, 2207, and 2208 are perpendicular to the <110>surface of the wafer and two planes 2209 and 2210 descend from thesurface at angles of 35.29 degrees. The two sloping planes 2209 and 2210meet in the line GH at the bottom of the cavity and are bounded by theperpendicular <111> planes. In general, the points G and H are not atthe bottom of the vertical lines descending from points A and D,respectively, where the vertical <111> planes 2205 and 2206 and thevertical <111> planes 2207 and 2208, respectively, meet.

FIG. 22H shows the hexagonal cavity 2204 in silicon wafer 2201 inoblique view. Vertical sides 2207 and 2208 and the sloping planes 2209and 2210 are visible in this view.

FIG. 22C is a vertical section taken through the line 1-1' in FIG. 22Bshowing portions of the vertical <111> plane 2207 descending from lineAB and the vertical plane 2208 descending from line FA in FIG. 5B and aportion of the sloping <111> plane 2209 descending from line BC in FIG.22B. FIG. 22D is a vertical section taken through the line 2-2' in FIG.22B. Line 2-2' is perpendicular to the line GH along which the sloping<111> planes 2209 and 2210 intersect. FIG. 22D shows the entire vertical<111> plane 2207 descending from the line AB and the entire verticalplane 2208 descending from line FA in FIG. 22B, which planes terminatethe V-groove formed by the sloping <111> planes 2209 and 2210 descendingfrom lines BC and EF, respectively, in FIG. 22B.

FIG. 22E illustrates an expanded view of an irregular rhomboidal cavity2211 of a type that can also be formed in a <110> silicon wafer like theone shown in FIG. 22A. The positions of the six lines in which thehexagonal cavity in FIG. 22B intersect the surface of the wafer are alsoillustrated for reference. The cavity 2211 may only be formed through ahole in a silicon dioxide film which has sides that are congruent withthe four lines IJ, JK, KL, and LI in which the four <111> planes of therhomboidal cavity intersect the surface of the silicon wafer. Of thesefour <111> planes, two <111> planes 2212 and 2213 are perpendicular tothe surface of the wafer, and two <111> planes 2214 and 2215 descendinto the wafer at an angle of 35.29 degrees with respect to the <110>surface of the wafer. The two sloping planes 2214 and 2215 meet in theline 2216 at the bottom of the cavity and are bounded by theperpendicular <111> planes. In general, the vertical <111> planes 2212and 2213 have the shape of an isosceles triangle.

FIG. 22F illustrates the opposite side of the <110> silicon wafer shownin FIG. 22A. FIG. 22G illustrates an expanded view of an irregularrhomboidal cavity 2217 that is the mirror image of the one shown in FIG.22E so that if the <110> surface shown in FIG. 22E and the <110> surfaceshown in FIG. 22G are brought into contact, the points I and N, J and M,K and P, and L and O may be aligned.

FIG. 23 and FIG. 24 show in perspective the opposite sides of arepresentative portion of the preferred embodiment of an integratedregenerative displacer chip in a <110> silicon wafer such as the oneshown in FIG. 5A. The dotted lines 2301 in FIG. 23 and 2401 in FIG. 24indicate that the chip may be extend an arbitrarily long distanceparallel to that line. The chip shown in FIG. 23 and FIG. 24 comprises aregenerator 2302 and 2402 with spaced, parallel walls in <111> planesperpendicular to the <110> silicon wafer in which the chip wasfabricated, a displacer diaphragm 2303 and 2403 in a <110> planeparallel to the <110> plane of the wafer in which the chip wasfabricated, a reticulated network of walls 2304 and 2404 in <111> planesperpendicular to and sloping at an angle of 35.29 degrees with respectto the <110> surface of the wafer in which the chip was fabricated, anda peripheral extension of the displacer diaphragm 2305 and 2405. Inoperation, the reticulated network of connecting walls 2304 and 2404anchor the displacer diaphragm 2303 and 2403 to the rest of thetransducer so that working fluid is pumped between the spaced parallelfins of the regenerator 2302 and 2402.

The reticulated network of <111> walls 2304 includes sloping <111> walls2307a and 2307b that connect the displacer diaphragm 2303 and itsextension 2305 to a <110> surface 2306 on the opposite side of the chip.To reduce the thermal cross section of the regenerative displacer in theaxial direction perpendicular to the plane of the chip, the silicon thatwas originally between the sloping <111> walls 2307a and 2307b has beenremoved to leave only the spaced, parallel, vertical <111> walls 2407. Asimilar reticulated network of <111> walls 2308 and 2408 reduces thecross sectional area at either side of the regenerator 2302 and 2402.

FIG. 25 and FIG. 26 illustrate an alternative embodiment of anintegrated regenerative displacer chip. In the embodiment shown in FIG.25 and FIG. 26, the reticulated network 2501 and 2601 of connectingvertical and sloping <111> walls are not laterally connected to thereticulated network 2502 and 2602 of connecting vertical and sloping<111> walls and the reticulated network 2503 and 2603 of connectingvertical and sloping <111> walls.

FIGS. 27A, 27B, and 27C show vertical sections through the preferredembodiment of the regenerative displacer chip shown in FIG. 23 and FIG.24, and through a mirror image of such a chip. The regenerativedisplacer chip and its mirror image may both be formed from oppositesides of <110> silicon wafers by exploiting the symmetriccrystallography of silicon described above. The sections shown in FIG.27A taken through the network of connecting walls illustrate that thesloping <111> planes 2701 and 2702 interconnect <110> surfaces 2703 and2704 on opposite sides of the chip, and that the resulting <110>surfaces 2705 and 2706 align with one another so that they can beinterfaced to one another for bonding.

The sections shown in FIG. 27B are taken through a portion of thedisplacer diaphragm 2707 laterally spaced from the regenerator, whereasthe sections shown in FIG. 27C are taken through the displacer diaphragm2708 and through one of the spaces 2709 between parallel fins of theregenerator.

FIG. 28A shows a vertical section through a regenerative displacercomprised of three regenerative displacer chips 2801, 2802, and 2803such as the one shown in FIG. 23 and FIG. 24 and three mirror images ofsuch chips 2804, 2805, and 2806 bonded together as a mechanicallyintegrated structure. The section shown in FIG. 28A taken through thereticulated network of connecting <281> walls shows that the resultingstructure rigidly connects the regenerative displacer to the adjacentsilicon chips 2807 and 2808. Other numbers of regenerative displacerwafers may also be used to comprise a regenerative displacer. Evacuationof some or all of these cells reduces the thermal conductivity of thestructure.

FIG. 28B shows another vertical section taken through the displacerdiaphragm of the preferred embodiment of the regenerative displacer.This section shows the expansion space 2809 of a regenerativethermomechanical transducer bounded by the silicon heat accepting plate2810 and the displacer diaphragm 2811, and the compression space 2812 ofthe transducer bounded by the heat rejecting plate 2813 and thedisplacer diaphragm 2814. The area of the displacer diaphragm 2811 islarger than the area of the displacer diaphragm 2814.

FIG. 28C shows another vertical section taken through the regeneratorand the displacer diaphragm of the preferred embodiment of theregenerative displacer. This section shows four networks of sloping<111> walls 2815, 2816, 2817, and 2818 that constitute part of thethermal conduction pathway between the heat ejecting plate 2819 and theheat accepting plate 2820.

FIGS. 29A, 29B, 29C, and 29D show vertical sections through thepreferred embodiment of the regenerative displacer chip shown in FIG. 23and FIG. 24, and through a mirror image of such a chip. The sectionsshown in FIG. 29A taken through the network of connecting wallsillustrate that the vertical <111> planes 2901 and 2902 interconnect<110> surfaces 2903 and 2904 on opposite sides of the chip, and that theresulting <110> surfaces 2905 and 2906 align with one another so thatthey can be interfaced to one another for bonding.

The sections shown in FIG. 29B are taken through a portion of thedisplacer diaphragm 2907 laterally spaced from the regenerator, whereasthe sections shown in FIG. 29C are taken through the displacer diaphragm2908 and through the network of walls 2909 adjacent to the regenerator.The section shown in FIG. 29D is taken through the displacer diaphragm2909 and through the parallel fins of the regenerator 2910.

The extended <110> planes 2903 in FIG. 29A, 2911 in FIG. 29B, 2909 and2912 in FIG. 29C, and 2913 and 2914 in FIG. 29D provide planar surfacesfor bonding to adjacent wafers.

FIG. 30A shows a vertical section through a regenerative displacercomprised of three regenerative displacer chips 3001, 3002, and 3003such as the one shown in FIG. 23 and FIG. 24 and three mirror images ofsuch chips 3004, 3005, and 3006 bonded together as a mechanicallyintegrated structure. The section shown in FIG. 30A taken through thenetwork of connecting <111> walls shows that the resulting structurerigidly connects the regenerative displacer to the adjacent siliconchips 3007 and 3008.

FIG. 30B shows another vertical section taken through the displacerdiaphragm of the preferred embodiment of the regenerative displacer.This section shows the expansion space 3009 of a regenerativethermomechanical transducer bounded by the silicon heat accepting plate3010 and the displacer diaphragm 3011, and the compression space 3012 ofthe transducer bounded by the heat rejecting plate 3013 and thedisplacer diaphragm 3014. The area of the displacer diaphragm 3011 islarger than the area of the displacer diaphragm 3014.

FIG. 30C shows another vertical section taken through the displacerdiaphragm 3015 and the network of walls adjacent to the regenerator3016.

FIG. 30D shows another vertical section taken through the regeneratorand the displacer diaphragm of the preferred embodiment of theregenerative displacer. This section shows four networks of sloping<111> walls 3017, 3018, 3019, and 3020 that constitute part of thethermal conduction pathway between the heat ejecting plate 3021 and theheat accepting plate 3022. FIG. 30D also shows the spaced, parallel finsof the regenerator 3023, of the heat ejecting heat exchanger 3024, andof the heat accepting heat exchanger 3025.

Fabrication Methods!

Embodiments of the present invention are preferably fabricated byadapting current planar integrated circuit fabrication processes to theproduction of these embodiments. With respect to the regenerativedisplacer chips, in particular, the integrated structures lendthemselves well to the fabrication of large numbers of multiplereplications as a unitary structure in a single <110> silicon wafer. Forexample, embodiments may be formed by forming a heat regenerator, adisplacer diaphragm, and a network of connecting walls as an integratedstructure by appropriately etching a <110> silicon wafer. Multiplicitiesof mirror images of these <110> wafers may be aligned, brought intointimate interfacial contact, and bonded together by fusion bonding orby eutectic bonding. The thermal conductivity of the axial thermalconduction pathway in this integrated structure may also be reduced byup to two orders of magnitude by totally oxidizing the axial thermalconduction pathway in a high pressure oxidation furnace. One or more ofthe resulting bonded regenerative displacer plates may then be bondedin-the-plane by similar means to heat accepting and heat ejecting heatexchanger plates to form a complete Stirling cycle or Vuilleumiertransducer. This method has the advantages of forming the regenerativedisplacer plate of a regenerative thermomechanical transducer fromsilicon wafers, like the heat accepting and heat ejecting plates, offorming the regenerator, the displacer, and the walls for connecting thetransducer plates as an integrated structure rather than as separatecomponents in separate pieces of material that then have to be joinedtogether, and of enabling the axial thermal conductivity of theregenerative displacer structure to be reduced after the structure ofthe regenerative displacer has been formed.

An hermetic seal for electrical leads to an internal heat source may beproduced by oxidizing silicon wafers that are to be joined, bydepositing and photolithographically defining electrical conductors onat least one of these wafers, by depositing an electrically insulatingmaterial over the metalized surface, by planarizing the resultingsurface, and then by aligning the two wafers and bringing them intointimate contact at a high temperature to bond the interfacial surfacestogether.

Conclusion!

From the above it is apparent that in embodiments of the presentinvention the entropy generating processes in oscillatory flow aregreatly reduced which allows the embodiment to operate at much higherfrequencies than previously thought possible for regenerativethermomechanical transducers and thus allowing a tiny machine to havepractical thermal pumping capacity and very desirable specific capacity.While prior art regenerative thermomechanical transducers have probablyoperated at the conventional lower frequencies with a Wolmersley numberbelow 5, the significance of the relationship between displacementamplitude and Mach number have never been associated with an opportunityto build small, high frequency machines.

The broad concept of the invention is the combination of passagesexhibiting a characteristic Wolmersley number below 5 and Mach numbersbelow 0.1 combined with a frequency of operation above 500 Hz.

Because of the dramatic increase in the thermal conductivity of siliconas temperatures decline, silicon makes an exceptionally desirablematerial for low temperature heat exchangers and heat conductingcomponents. At the same time, silicon is an ideal substrate material forthe attachment of silicon chips since there is no difference in thermalcoefficient of expansion between the substrate and the chip, which, isthere were, would otherwise tend to cause detachment or separation underextreme temperature excursions that occur in a cryocooler. Since siliconis the most common material in which electronic circuits are fabricated,silicon offers the possibility of fabricating circuits into thestructure of the microrefrigerator's heat exchanger itself. Furthermore,since silicon can be oxidized into silicon dioxide, which has aconsiderably lower thermal conductivity than silicon, after having beenformed into complex structures, silicon is an ideal material for formingregenerative displacers. These regenerative displacers may then be usedto implement free-piston thermomechanical transducers that require fewerand in some cases no electromechanical transducers for energyconversion.

The small size and high frequency of this machine allows the machine tooperate in near-isothermal conditions, unlike the less energy efficientadiabatic conditions in previous, larger, lower frequency machines.

While certain preferred embodiments of the present invention have beendisclosed in detail, it is to be understood that various modificationsmay be adopted without departing from the spirit of the invention orscope of the following claims.

We claim:
 1. A regenerative displacer component for a thermomechanicaltransducer, the displacer component comprising an integrated bodyforming a heat regenerator in a working fluid displacer the workingfluid displacer reciprocating to alternately transport a working fluidthrough the heat regenerator, and a reticulated network of wallssurrounding the working fluid displacer for supporting the working fluiddisplacer and for connecting the working fluid displacer to other partsof the thermomechanical transducer.
 2. A regenerative displacercomponent in accordance with claim 1 wherein the integrated bodycomprises a low thermal conductivity material.
 3. A regenerativedisplacer component in accordance with claim 2 wherein said low thermalconductivity material is silicon dioxide.
 4. A regenerative displacercomponent in accordance with claim 1 wherein the displacer componentcomprises a plurality of parallel, axially spaced annular diaphragmsconnected centrally to the working fluid displacer and peripherally tothe walls.
 5. A regenerative displacer component in accordance withclaim 4 wherein said annular diaphragms are flat sheets.
 6. Aregenerative displacer component in accordance with claim 4 wherein saidannular diaphragms have annular corrugations.
 7. A regenerativedisplacer component in accordance with claim 4 wherein the working fluiddisplacer comprises an axially perforate matrix forming the regeneratorand having continuously connected voids providing axial working fluidflow paths through the regenerator.
 8. A regenerative displacercomponent in accordance with claim 7 wherein the perforate matrixcomprises a plurality of parallel, spaced planar walls defining passagesbetween the walls having across-sectional aspect ratio greater thansubstantially
 8. 9. A regenerative displacer component in accordancewith claim 4 wherein the regenerative displacer component ismechanically resonant at substantially the operating frequency of thethermomechanical transducer of which it is a part.
 10. A regenerativedisplacer component in accordance with claim 9 wherein the walls definea work space of the transducer and the work space comprises a gas springhaving a characteristic spring constant acting upon the displacer andwherein the working fluid displacer mass and displacer diaphragm springconstant together with the gas spring constant are selected to resonatethe working fluid displacer at substantially the operating frequency ofthe transducer.
 11. A regenerative displacer component in accordancewith claim 1 wherein the walls comprise a reticulated network of cells.12. A regenerative displacer component in accordance with claim 11wherein the reticulated network of cells form a sealed pressure vessel.13. A regenerative displacer component in accordance with claim 11wherein at least some of the cells are evacuated.
 14. A regenerativedisplacer component in accordance with claim 1 wherein lateral surfacesof the regenerator form a sealed pressure vessel.